The present invention relates to films made from vinylidene chloride polymers or copolymers; more particularly to a method, composition, and film for controlling the oxygen and carbon dioxide transmission and stickiness, and enhancing the free flowability and thermal stability of monolayer and multilayer packaging films having a plasticized layer of vinylidene chloride copolymer film.
Thermoplastic packaging films made of vinylidene chloride copolymer, here referred to as "saran", have long been used to package food such as cheese, fresh meat, etc. Saran is a good barrier to the transmission of oxygen.
Methods of producing a multilayer film having a layer of saran are disclosed in U.S. Pat. No. 4,112,181 (Baird) and U.S. Pat. No. 3,741,253 (Brax et al), both incorporated herein by reference. Saran is typically plasticized and stabilized for purposes of extrusion. One example is epichlorohydrin/bisphenol A, and epoxy resin, and 2-ethyl hexyl diphenyl phosphate.
U.S. Pat. No. 5,202,188 discloses a vinylidene chloride composition comprising 1 to 4% plasticizer, 1 to 4% acrylate/styrene polymer, and the balance of 92 to 98% comprising a vinylidene chloride copolymer. Although this film has proven commercially useful in many packaging applications requiring high oxygen barrier, it does not address the needs of packaging applications such as gas (CO.sub.2) producing cheeses, and produce (vegetables) where a moderate O.sub.2 barrier combined with relatively high CO.sub.2 transmission is often required.
A commercial composition currently used to make film for packaging gassing cheese includes 100 parts VDC/VC plus three liquid plasticizers: five parts of Plastolein 9759-A (a polymeric condensation product of azelaic acid and 1,3-butandiol), three parts of Plasthall HA7A, a polymeric plasticizer of polyester of adipic acid and propylene glycol, and one part Plas-chek 775, an epoxidized soybean oil. It has been found that blends with liquid plasticizers do not free flow very well and thus require a grinding step before extrusion. This process, coupled with some migration of the liquid additives out of the VDC/VC matrix, causes inconsistency, i.e. lack of homogeneity in the composition of the resulting blend. This inconsistency in turn leads to inconsistency in the O.sub.2 and CO.sub.2 transmission rates of packaging films made from the blend. This is important, because consistent O.sub.2 and CO.sub.2 transmission rates are needed to insure a commercially acceptable package system. When these gas rates can be controlled predictably, the amount of the additives can be adjusted to optimize the transmission rates for the intended end use.
Some cheese products are produced in such a way that the final cheese product emits a significant amount of carbon dioxide over time. In such cases, it is often desirable to provide a packaging material which is characterized by a relatively low oxygen transmission rate to protect the cheese product from oxidative degradation in quality or shelf life. It is also desirable to have a sufficiently high carbon dioxide transmission to allow the naturally generated CO.sub.2 to escape from the package, and avoid undesirable ballooning of the package.
The present invention provides a composition, and film, with a higher total mount of plasticizcr and stabilizer than that shown in the art, and therefore a film suitable for packaging applications where O.sub.2 and CO.sub.2 transmission rates are important. In preferred embodiments, the invention provides not only a higher amount of plasticizer and stabilizer, but also a relatively lower amount of liquid plasticizer in the blend. The resulting improvements in free flowability save an extra grinding step in preparing the composition, and also offer a more controlled, consistent O.sub.2 and CO.sub.2 transmission.
The present invention also provides good thermal stability for saran formulations, and they do not degrade to any significant extent during extrusion. Methods for determining stability are well known, e.g. as described in U.S. Pat. No. 5,202,188 incorporated herein by reference.